Osteoporosis affects 10 million Americans and another 34 million are osteopenic and at risk for developing osteoporosis. Bone fractures, the most important complication of osteoporosis, cause substantial morbidity and mortality in the aging population as well as significant socio-economic cost. Since the 1960's, bisphosphonate drug therapy has produced modest clinical benefits such as improved bone density and reduced fracture risk by slowing osteoclastic bone resorption. To this date, all marketed therapies for osteoporosis as well as the majority of potential new treatments under clinical investigation aim to reduce the level of bone resorption in osteoporotic patients. Anti-resorptive drug therapy has been most effective in treating early and mild cases of the disease, unlike severe osteoporosis, where a massive loss of bone mineral density has already occurred. New strategies are urgently needed for intervention in osteoporosis, particularly in severe cases of the disease, including targeting mechanisms of bone formation that have previously been unexplored that can safely promote anabolic bone growth. Presently, there is only one FDA approved bone anabolic agent, Forteo (teriparatide) that confers significant clinical benefits in osteoporosis, but its use is severely restricted due to safety concerns. Multipotent mesenchymal stem cells (MSCs) are precursors of a variety of cell types, including osteoblasts and adipocytes. Formation of new bone is driven by osteoblastic differentiation of MSCs, a process that can be disrupted by age and other factors in favor of adipogenesis. Parhami et al. discovered that specific naturally-occurring oxysterols induce osteogenesis when applied to MSCs while inhibiting their adipogenesis. Recently, we have characterized a new series of semi-synthetic analogues of the natural oxysterols with improved properties. Our most advanced compound, OXY133, displays increased potency for osteogenic differentiation in vitro, and stimulates robust localized bone formation in vivo in a rat and rabbit spine fusion model. Here we propose to begin evaluating Oxy133 as a bone anabolic agent in the context of systemic administration and bone targeting by taking advantage of the well-known affinity of bisphosphonates for selectively binding to bone mineral. Our strategy involves conjugation of OXY133 to a known bisphosphonate, alendronic acid, which has previously been used as a bone targeting agent, using tunable linkers for controlled release. We predict that systemic dosing of such conjugates will result in their selective deposition in bone followed by enzymatic linker hydrolysis and release of the osteogenic agent, OXY133, at controlled rates into the bone tissue. We propose to perform studies as part of three Specific Aims that involve: 1) design & synthesis of Oxy133-bisphosphonate conjugated analogues, 2) examination of the hydroxyapatite binding capacity of these analogues, and 3) assessment of their osteogenic activity. Information obtained from these studies will provide the rationale for future investigation of the therapeutic effects of Oxy133-bisphosphonate analogues in animal models of osteoporosis.